I see this practice done in a lot of pictures of modern AFVs in the Middle East. What is the purpose of the water bottles on the antenna? Is it to add weight for some reason?

Tabb,

It's a common way to carry chem lights to ID friendlies at night. The empty bottles provide a simple, clear, waterproof container that can be taped or attached to the aerial - The chem lights can be replaced quickly and easily just by unscrewing the cap of the bottle.

In 1/35 there are a couple of simple ways to replicate the water bottle/chem light.

For a previous project I used a cut-down Academy water bottle from their generic modern clear sprue that they supply in some kits (cut down from the '2 litre' to represent a '1 litre' bottle).

I drilled a hole down into the bottle from the cap end and then filled it with paint (carefully, to avoid getting it on the outside of the bottle). Finally I painted the 'cap' in blue. The whole thing sits upside down on the aerial and was attached with a couple of thin strips of Tamiya tape (pre-painted and cut to length)